Radio frequency (RF) antennas are used to receive and/or radiate RF signals.
An effective antenna for use in transmission will exhibit an acceptably low standing wave ratio (SWR) at the frequencies of interest, and will present a reasonably good impedance match to the output of the transmitter, typically 50Ω to 75Ω. While some antenna designs such as beams exhibit directionality, i.e., more antenna gain in one direction compared to another, in many applications it is desired that the pattern of radiation from the antenna be omnidirectional. Further it is often desired that the antenna not require ground radials. Ground radials undesirably increase antenna wind load thus lessening robustness and portability, and increase manufacturing cost.
Many innovations in antenna design have come from the amateur radio community. Pioneer work in the area of so-called fractal antenna has been accomplished by Nathan Cohen (W1IR, W1YW) of Belmont, Mass., e.g., U.S. Pat. Nos. 6,104,349, 6,127,977, 6,140,975, 6,445,352, 7,019,695, and 7,701,396, among others.
Another innovation in multi-band antenna design is depicted in FIGS. 1A and 1B, namely the so-called Don Johnson screwdriver antenna, invented in March 1991 and named after its late inventor Don Johnson (W6AAQ) of Esparta, Calif. Overall antenna 10 includes a whip portion 20, typically 3′ to perhaps 8′ in length, mounted to make electrical connection with the upper end of an inductor 30. Inductor 30 typically is formed about a non-conductive cylinder of perhaps 2″ diameter and perhaps 12″ length. The upper portion of housing 40 includes conductive finger stock that presses against inductor 30, effectively grounding to housing 40 all portions of inductor 40 that are within the housing. Inductor 30 and the cylinder it is formed upon can be urged vertically upward and downward within a metal cylinder housing 40 to alter magnitude of the effective inductance protruding from housing 40. A threaded rotatable shaft 80 is connected between the lower end of the inductor 30 cylinder and the rotatable shaft of a small DC motor 70. Motor 70 is typically a motor from an electric screwdriver, hence the “screwdriver” name for the antenna. Two wires from motor 70 can be applied to a plus or a minus polarity DC voltage, to cause motor 70 to rotate clockwise or counterclockwise, causing more or less of inductor 30 to lie within housing 40, which is to say to decrease or increase magnitude of the effective inductance protruding from the housing. A matching inductor 50 is formed at the base of the antenna and a length of typically 50Ω coaxial cable 60 is connected as shown. The other end of coaxial cable 60 will typically go to a transceiver, or transmitter, or receiver. Tuning antenna 10 simply involves applying plus or minus DC voltage to motor 70 to mechanically resonate the antenna to a desired frequency range. Antenna 10 can be tuned by first setting the transceiver (or receiver) to a desired frequency and then adjusting the effective length of inductor 30 by rotating motor 70 in the proper direction (by applying plus or minus voltage to motor 70) to resonate the antenna, as evidenced by a peak in amplitude of received signals.
In FIG. 1A, DC voltage has been applied to motor 70 to rotate nearly all of inductor 30 into housing 40. The effect at RF frequencies is that only the portion of inductor 30 protruding from housing 40 functions as an inductor. The antenna operates as a center loaded device whose resonance is determined primarily by the effective inductance 30 and the whip 20. In FIG. 1B, DC voltage was applied to motor 70 to rotate threaded shaft 80 such that more of inductor 30 can now resonant with whip 20 such that the additional effective inductance lowers the resonant frequency of the overall antenna. Advantageously the antenna can operate continuously within a very wide range of frequencies, merely by applying DC voltage to motor 70 to cause more or less inductance to be used. In practice, many thousands of Don Johnson (W6AAQ) screwdriver antennas have been used worldwide with great success over frequencies ranging from as low as about 3.5 MHz to as high as perhaps 144 MHz.
In other applications, especially higher frequency applications, a less mechanical antenna may be desired, especially for considerations of cost and ease of construction. In the radio amateur community, high frequency bands of interest include VHF (2 m range wavelengths, typically about 144 MHz to about 148 MHz), UHF (70 cm range wavelengths, typically about 420 MHz to about 450 MHz), and intermediate to VHF and UHF, the 1.25 m range wavelengths (typically about 222 MHz to about 225 MHz). One common type of antenna, especially for VHF (2 m range wavelengths) and/or UHF (70 cm range wavelengths), is the so-called collinear antenna. A collinear antenna is an array of at least two dipole antennas, configured such that every element of each dipole is an extension, relative to a longitudinal antenna axis, of the other dipoles in the array. Collinear antennas can exhibit gain over an isotropic radiator.
FIG. 2A depicts an antenna 90 comprising collinear elements 100, 110, 120, used with at least four antenna base-mounted quarter-wavelength radials 130 that function as a ground plane. Preferably coaxial cable 60 is coupled to antenna 90, with the other end of coaxial cable 60 coupled to a transceiver, a transmitter, or a receiver (not shown). Lowermost element 100 in FIG. 2A is a quarter-wavelength at the nominal frequency of interest. Intermediate element 110 is coupled to act as a half-wave delay element, and uppermost radiating element 120 preferably has a length equal to a half-wave. The various elements 100, 110, 120 can be fabricated from lengths of coaxial cable, whose center conductor is indicated by phantom lines, and whose outer shield conductor is indicated by solid lines on either side of the center conductor. Note that the collinear arrangement alternates electrical connection between the center conductor of an element and the outer conductor.
In FIG. 2A, if one tried to use quarter-wavelength element 100 with an extension half-wavelength (i.e., center-conductor to center-conductor, shield-to-shield), no additional gain would result due to phase cancellation of radiation in the quarter-wave and half-wave elements. FIG. 2B depicts voltage amplitude versus phase for the various elements of antenna 90. As confirmed by FIG. 2B, non-radiating half-wave delay element 110 provides the desired ground reference function. This results from coupling the shielded outer conductor of element 100 to the inner conductor of element 110, which inner conductor acts as a ground reference. Note at the base of antenna 90 that radials 130 are also coupled to this ground reference via the center conductor of element 100. As shown in FIG. 2A, after a quarter-wavelength at the junction of elements 100 and 110, the shield and inner conductor are swapped. At its upper end, element 110 is coupled to the lower end of half-wave coaxial element 120, again by swapping of center conductor and shield outer conductor. As the radiated radio frequency energy exits the upper end of element 120 it is back in phase with quarter-wavelength radiating element 100. If desired additional elements, i.e., another triplet of elements 100, 120, 120 could be added atop present uppermost element 120 in collinear fashion. However a point of diminishing returns effectively occurs at about four elements in that marginal further increase in gain does not warrant the cost of the additional elements.
Disadvantageously, antenna 90 requires several, typically at least four, quarter-wavelength radials 130, preferably bent downward at an angle of perhaps 45° to establish an RF ground. As noted, an RF ground reference node exists at the junction of radials 130 and the outer shield of coaxial cable 60. Radials often require machining to properly make good electrical connection at the base of antenna 90. In practice stainless steel radials are preferred for reasons of strength and electrical contact over less expensive aluminum radials. The presence of radials impacts the robustness of the antenna design. Radials can easily break off in the presence of strong winds, or by birds perching on the radials. If the radials are on the ground, they may be damaged from being walked upon. Further, the electrical conductivity between the radials and the shield of coaxial cable 60 will inevitably deteriorate over time.
FIG. 3 depicts an attempt in the prior art to eliminate radials by using a quarter-wave sleeve. Referring to FIG. 3, antenna 140 has at its base a quarter-wave element 100, then a half-wave delay element 110, above which is disposed an upper half-wave radiating element 120. These collinear elements 100, 110, 120 in antenna 140 are configured similarly to the same elements in antenna 90 in FIG. 2A, and are made from segments of coaxial cable. Rather than employ radials (as in FIG. 2A), antenna 90 employs a conductive quarter-wavelength sleeve 150 to implement an effective quarter-wavelength foldback and RF ground reference. The term “foldback” is used in that sleeve 150 covers a portion of the connecting coaxial cable 60. Sleeve 150 is commonly made of conductive brass or copper pipe, and the connection to coaxial cable 60 is typically made within the sleeve. This configuration advantageously gains robustness by eliminating radials, and exhibits a slightly lower angle of radiation that can add to transmitting and receiving range of the antenna. However the sleeve configuration can make it difficult to achieve desired low SWR due to inherent coupling between the outer shield conductor of coaxial cable 60 and the wall of sleeve 150.
FIG. 4 depicts yet another attempt in the prior art to implement an omnidirectional antenna without radials. As shown in FIG. 4, a portion of this antenna looks like the letter “J”, and this configuration is sometimes referred to as a “Super-J” antenna. A full description of antenna 160 may be found in the ARRL Antenna Handbook, 19th ed., chapter 16, pp. 24-27, “The Supper J Maritime Antenna.” Referring to FIG. 4, the lower end of antenna 160 is a quarter wavelength matching element 170, typically 300 twinlead, whose two leads or wires are shorted together at the bottom 180. The RF impedance at the shorted bottom 180 is of course 0Ω, but at a distance Δ above bottom 180, the RF impedance will be close to the impedance of coaxial cable 60 to be a good match, e.g., 50Ω or so. The upper end of quarter-wavelength matching element 170 is coupled to a half-wave radiating element 190, as the upper end of quarter-wavelength matching element 170, and either end of half-wave radiating element 190 are both RF high impedances.
Referring still to FIG. 4, note that elements 190 and 210 are disposed vertically and are RF radiating elements. By contrast, the upper end of half-wave radiating element 190 is coupled to a horizontally disposed delay element 200. Delay element 200 comprises two parallel quarter-wavelength element coupled in a horizontally-disposed “U”-shaped configuration. The horizontally polarized RF energy associated with the lower and with the upper elements of delay element 200 are 90° out-of-phase with respect to each other and thus cancel one another. Ideally the phase delay and radiation patterns associated with element 200 would be perfectly out-of-phase, but in practice some phase error and associated antenna inefficiency will exist. “U”-shaped delay element 200 may be thought of as contributing an outgoing lower quarter-wavelength delay and an incoming quarter-wavelength delay. The net result is that these horizontally disposed elements represent an effective half-wave delay element 200. The upper portion of “U”-shaped delay element 200 is connected to the lower end of a vertically disposed (and vertically radiating) half-wave radiating element 210. In this fashion the antenna of FIG. 4 implements the functional equivalent of the ready access to ground that was present in the antenna of FIG. 2A. The desired overall half-wavelength delay with desired non-radiating characteristics for 180° of the phase waveform is achieved by “U”-shaped element 200.
Regrettably, antenna 160 is not robust in that delay element 200 projects out horizontally from the vertical antenna into the environment, and is difficult to reliably fasten between radiating elements 190 and 210. Alternatively some designs also seek to achieve phase delay with inductor-capacitor (LC) components rather than with an element 200. However such solutions are not optimum because losses and tolerance changes in the L and C components vary over time, which can reduce effectiveness of the desired delay function.
FIG. 5 depicts a so-called conventional mono-band “J-pole” antenna that can be fabricated from a single length of 300Ω twin lead cable, comprising LEAD 1 and LEAD 2, that has a gap or notch cut in one lead (LEAD 1). The lowermost end of antenna 220 has a short 180 between LEAD 1 and LEAD 2 to establish a 0Ω region, adjacent to which is a quarter-wavelength impedance matching element 230, sometimes referred to as a shorting stub element. Impedance matching element 230 is passive in that it does not radiate RF energy. A perhaps 0.25″ gap or notch 240 is formed in LEAD 1 to separate element 230 from the remainder of antenna 220. A half-wave RF radiating element 250 is formed above gap 240 in LEAD 1 and above matching element 230 in LEAD 2. The uppermost end of antenna 220 is open ended, or high impedance, and the lowermost end by virtue of the short is 0Ω. Experimentally a pair of low impedance, e.g., 50Ω, feedpoints are found a distance Δ (typically about 1.25″) above the 0Ω short 180. Coaxial cable 60 (typically RG174A) center conductor is connected to one feedpoint and the coaxial cable braid shield is connected to the other feedpoint.
Antenna 220 in FIG. 5 may be cut or sized for the 144 MHz VHF band for use within protective 0.75″ O.D. 200 PSI PVC pipe, with the detuning effect of the PVC pipe being accounted for in the following dimensions. As such for VHF half wavelength RF radiating element 250 will measure about 37.25″, gap 240 will be 0.25″, distance Δ will be about 1.25″, and quarter wavelength matching element 230 will be about 16″. If antenna 220 were cut or sized for the 440 MHz UHF band within the same protective PVC pipe, half wavelength RF radiator 250 would measure about 12″, gap 240 will be 0.25″, distance Δ will be about 0.5″, and quarter wavelength matching element 230 will be about 5″, with RG174A coaxial as a preferred cable 60. Of course antenna 220 in FIG. 5 could be scaled to operate in the 220 MHz band, in which case half-wavelength radiating element 250 would be about 26″ in length, quarter-wavelength impedance matching element 230 would be about 9.75″, the latter 0.75″ being the dimension of Δ. These exemplary dimensions assume the finished antenna will be mounted with 0.75″ diam. 200 PSI PVC pipe. The term “J-pole” arises from the “J-shape” defined by the LEAD 1 portion of region 230 including the 0Ω short, including Lead 2 extending to the top of the antenna.
Monoband J-pole 220 in FIG. 5 operates as a half wavelength vertical end-fed dipole antenna. In a conventional half-wave antenna, the antenna ends are high impedance and the antenna center including the feedpoints is low impedance. However in the half wavelength J-pole configuration of FIG. 5 the inclusion of the passive quarter wavelength impedance matching section 230 enables end matching to the antenna with an approximately 50Ω feedpoint pair. If antenna 220 is mass produced, it suffices to measure distance Δ on a prototype antenna using an antenna analyzer, and to fine tune location of gap 240, and then to replicate the prototype antenna in quantity. Vertical end-fed monoband J-pole dipole antenna 220 in FIG. 5 has a radiation pattern close to an ideal dipole, thanks to end-fed coaxial cable 60 being in-line with the antenna axis or length. By contrast, in a conventional center-fed vertical dipole, the coaxial cable connects to the center of the antenna at 90° to the axis or length of the antenna, and thus distorts the dipole radiation pattern. A well-designed J-pole antenna 220 is a good half-wave radiator that provides about 2.1 dB gain over an isotropic radiator, but no gain relative to an ideal half-wave antenna.
It will be appreciated that monoband J-pole antenna 220 is omni-directional, inexpensive to fabricate, and requires no radials. In practice the antenna can be inserted within a length of UV-resistant PVC pipe that is sealed at the top and bottom, to provide a robust configuration with relatively low wind resistance. Understandably the dimensions of the J-pole antenna will be adjusted somewhat to compensate for the velocity factor effect of the surrounding PVC pipe upon the antenna characteristics in open air, to avoid detuning. In practice J-pole antenna 220 can achieve about a 1.5 dB gain improvement over a quarter wavelength ground plane antenna because it is a true half wavelength antenna. In a conventional ground plane antenna such as was described in FIG. 2A, ground radials 130 were necessary to act a counter element (e.g., ground or earth). With radials bent downward from say 0° (i.e., horizontal) to about 45°, and disadvantageously a relatively high angle of radiation will result. By contrast, monoband J-pole antenna 220 has no radials and advantageously exhibits a lower angle of radiation, which results in a gain of about 1.5 dB in the horizontal plane relative to a conventional ground plane antenna.
FIG. 6A depicts the DBJ-1 (WB6IQN) dual-band J-pole antenna 260, invented by applicant herein, which resonates and radiates efficiently as a half-wavelength vertical antenna on each of the VHF and UHF bands, using a single feedline, without need for a ground. Referring to FIG. 6A, at the very bottom region 180 of antenna 260 the twinlead (comprising LEAD 1, LEAD 2 with the plastic insulating material not shown) is shorted together to form a 0Ω region. However above this short at a distance Δ (typically about 1.25″) a low impedance (preferably about 50Ω) feedpoint pair can be located. Coaxial cable 60, e.g., RG174A, is attached to the antenna at this feedpoint pair and preferably exits antenna 260 collinearly rather than perpendicular to the antenna longitudinal axis or length, to reduce interference with the antenna RF radiation pattern.
Referring still to FIG. 6A, immediately above the low impedance feedpoints antenna is twinlead extending upward a distance l1 (typically about 16″), which together with the small distance Δ (about 1.25″) functions as a passive, non-radiating impedance transformer 262. At VHF impedance transformer 262 functions as a quarter wavelength stub matching transformer but at UHF functions as a three-quarter wavelength stub matching transformer, electrically the series sum of a quarter-wavelength and a half-wavelength. As it is non-radiating and passive, impedance transformer 262 does not interfere with UHF RF radiating section 264 or with VHF RF radiating section 268 of antenna 260. A notch or gap 240 of length l2 (about 0.25″) is formed in LEAD 1 to separate impedance matching stub region 262 from the remaining regions 264, 266, 268 of the antenna above.
Continuing upward in FIG. 6A, element 264 is the UHF half wavelength radiating portion of antenna 260 and comprises length l3 (about 11.25″) of twinlead. As will now be described region 266 functions as a mechanism to decouple the upper end of UHF half wavelength radiator 260 from region 266 and region 268. In theory such UHF decoupling mechanism might be a self-powered SPST relay that sensed when to open (during UHF triband antenna use) and when to close (during non-UHF triband use). In theory such UHF decoupling mechanism might be an ideal inductor in parallel with an ideal capacitor, the inductor-capacitor resonating in the UHF band, i.e., exhibiting infinitely high impedance. But in practice such relays do not exist, and perfect inductors and capacitor (which components have no loss, and do not drift in value) do not exist.
Applicant found a suitable, lightweight and inexpensive UHF decoupling mechanism to be the quarter wavelength stub 266, shown in FIG. 6A. UHF decoupling stub 266 comprises a length l4, a quarter wavelength at UHF (about 4.25″), of preferably RG174A coaxial cable disposed in LEAD 2 to terminate or isolate UHF radiating region 264 from VHF radiating region 268 above at UHF frequencies. At the upper end UHF decoupling stub 266 has its shield and center conductor soldered together to form a zero impedance, which at a UHF quarter wavelength transforms to an open or high impedance at the lower end or bottom of the stub. At the open lower end of stub 266, the center conductor of the coaxial length comprising the stub is soldered to the region of LEAD 2 opposing gap 240, and the braid shield is left floating. At UHF frequencies, UHF decoupling stub 266 presents an open circuit (high impedance) but at VHF frequencies acts as a closed circuit, albeit with small inductance. Above UHF decoupling stub 266 is a length l5 (about 17″) of twinlead that coupled with length l4 and length l3 of element 264 comprises the VHF radiating region 268 of antenna 260. It will be appreciated that UHF decoupling stub 266 acts somewhat as a switch that ideally is closed at VHF frequencies to allow lengths l2+l3+l4+l5 combine to function as a half-wave RF VHF radiator 268, but at UHF frequencies acts ideally as an open switch that decouples regions 266 and 268 from region 264. Region 264+l2 then act as a half-wavelength RF UHF radiator. On the other hand, at VHF frequencies decoupling stub 266 acts like a closed switch, and antenna regions 268 and 264 (and the length of decoupling stub 266) and the slight contribution from l2 combine to form a half wavelength at VHF. Consequently antenna 260 resonates well and radiates RF well at both VHF and UHF frequencies.
DBJ-1 antenna 260 as shown in FIG. 6A has enjoyed great acceptance with the radio amateur community, as well as with numerous emergency services agencies, federal, state, and local, that rely upon VHF/UHF communications devices. More than 10,000 DBJ-1 antennas have been constructed and deployed in the U.S. and abroad. The antenna of FIG. 6A may be fabricated from 300Ω twinlead, and fed with RG174A coaxial cable, cable 60, preferably coupling to the antenna along the antenna longitudinal axis, to minimize interference with the antenna RF radiation pattern. Twinlead fabrication of antenna 260 contributed to low manufacturing cost and portability. The twinlead portion of the antenna is small and lightweight, and can be mailed to a user. If the antenna will be used mounted within a specified type of PVC tubing, the antenna twinlead (LEAD 1, LEAD 2) is pre-cut during fabrication, taking into account the PVC tubing characteristics. The pre-cut twinlead is mailed to the user with instructions to buy the specified PVC tubing locally, and mount the antenna within. A complete description of this antenna is set forth in the article “A Dual Band VHF-UHF Single Feedline J-Pole”, QST, February 2003, vol. 87, no. 2, pages 38-40, by E. Fong, applicant herein. Further details regarding the design of antenna 220 may be found at QST magazine, February 2003, pp 38-401, E. Fong (WB6IQN), “The DBJ-1: A VHF-UHF Dual-Band J-Pole”, and QST magazine, March 2007, E. Fong (WB6IQN), “The DBJ-2: A Portable VHF-UHF Roll-up J-pole Antenna for ARES”.
Thus as used herein, the term DBJ-1 dual-band J-pole antenna is understood to refer to VHF-UHF twinlead antenna 260 as depicted and described above with reference to FIG. 6A. It will be appreciated that at UHF the radiating element is the lead 2 portion l5+l2, and that the the lead 1 portion above gap 240 is simply floating, does not radiate, and in fact could be removed. Similarly it will be appreciated that at VHF the radiating element is the lead 1 portion of l5+stub 366+the lead 1 portion of l3+l2, and that the lead 1 portion above gap 240 is simply floating, does not radiating, and could be removed. Thus above the level of l1, the twinlead comprising lead 1 and lead 2 could be replaced by a single wire denoted lead 2, with lead 1 being completely removed.
FIG. 6B depicts a fundamental and a third harmonic radiation pattern for a dual band J-pole such as shown in FIG. 6A, depicting radiation performance with and without UHF decoupling stub 266. In FIG. 6B, if the UHF decoupling stub were absent, radiation is the third harmonic pattern, a somewhat distorted butterfly pattern. This distortion results because the third harmonic of VHF is UHF, and has the undesired characteristic that perhaps 75% of the radiation emanates up into the sky. The fundamental radiation pattern, by contrast, results when UHF decoupling stub is used, as in antenna 260, and is a clean half wavelength dipole radiation pattern.
FIG. 7 depicts a prior art attempt to implement a triband antenna operable in the in the 2 m (144 MHz) VHF band, the 1.25 m (220 MHz) intermediate VHF-UHF band, and the 70 cm (440 MHz) UHF band. Antenna 280 in FIG. 7 is a stacked triband antenna described by J. L. Harris “A VHF/UHF) 3 Band Mobile Antenna”, QST Magazine, February 1980. Antenna 280 is essentially three parallel J-pole antennas, one antenna for each of the three bands, comprising a quarter-wave wavelength stub 282 for 146 MHz, a quarter-wavelength stub 284 for 220 MHz, and a quarter-wavelength stub 286 for 445 MHz, all fabricated using soldered-together 0.5″ O.D. copper plumbing pipe and copper plumbing elbow joints. These copper quarter-wavelength stubs are soldered to a vertical copper member 288, that comprises the main body of triband antenna 280.
Within hollow member 288 three lengths of coaxial cable such as 60 run vertically. One length has its center lead (shown in phantom) soldered to the shell of stub 286, and has its shield soldered to the shell of member 280 adjacent the entry hole. A second length of coaxial cable has its center lead soldered to the shell of stub 284 and its shield soldered to member 280 adjacent the entry hole. A third length of coaxial cable has its center lead soldered to the shell of stub 282, and has its shield soldered to member 280 adjacent the entry hole. At the bottom of antenna 280, all three center leads from the three coaxial cables are soldered together and to the center lead of feed coaxial cable 60, RG174A or the like. The shield of feed coaxial cable 60 is soldered to the shell of member 288 at the bottom of antenna 280. At the relevant resonant frequency band, each quarter-wave copper stub presents an approximately 50Ω impedance. Unfortunately antenna 280 in FIG. 7 lacks a mechanism to decouple the UHF radiator portion of the antenna from the VHF radiator portion, e.g., a mechanism serving the role of UHF decoupling stub 266 in DBJ-1 antenna 260 in FIG. 6A. Consequently because it is harmonically related, the VHF portion of antenna 280 will resonant at UHF frequencies, which disturbs the antenna radiation pattern at UHF. Another shortcoming is that triband antenna 280 in FIG. 7 is not especially robust, especially with the three small entry holes into the main vertical member, through which water can enter to the detriment of the center lead connections to coaxial cable 60 at the antenna bottom. Robustness is compromised by the inherent weakness of solder joints, especially in inclement weather. The overall antenna is about 1.6 m from top-to-bottom, is rigid but fragile, and is not readily shippable. However one advantage of the copper tubing fabrication is that the effective cross-section of the antenna is far greater than if twinlead were used. Consequently a copper tubing J-pole can exhibit greater bandwidth than a J-pole made of very narrow wire twinlead. However on balance, the cost, weight, lack of robustness, and difficulty in transporting a copper J-pole mitigate against using such material.
What is needed is an inexpensive, readily fabricated triband antenna that provides performance commensurate with a half wavelength vertical antenna on each band, has a virtual ground requiring no radials, provides independent adjustment, if needed, on each band without substantially affecting performance on the remaining bands. Such antenna should be collinear in form factor, robust, radiate omni-directionally, should be lightweight and inexpensive to fabricate. Further the antenna should be readily shippable and readily deployable in portable applications, and should be less than about 1.7 m in length. Finally, there should be a single antenna connection port common to all three bands such that a single external coaxial cable can be connected to the triband antenna for operation at any or all of the three bands.
The present invention provides such an antenna.